Pelvic Avulsion Fractures

A pelvic avulsion fracture is the injury in which a muscle-tendon unit pulls its apophyseal attachment off the pelvis. It occurs almost exclusively in adolescents, because the unfused apophysis is weaker than the muscle-tendon unit attached to it.

Who. Adolescents aged 14-17, before apophyseal closure, and males more often than females. The sports are sprinting, soccer, gymnastics, hurdling and baseball. Bilateral injuries are rare.

Where. The common sites are the ASIS, AIIS, ischial tuberosity, iliac crest and lesser trochanter; the muscle and mechanism for each are mapped under Anatomy.

The apophysis. An apophysis is a secondary ossification centre for a muscle attachment. Its cartilaginous growth plate is weaker than bone, muscle or tendon.

Mechanism. A sudden, explosive contraction against resistance, or an eccentric load in which the muscle lengthens under tension (the hurdler's hamstrings), avulses the apophysis before the muscle or tendon gives way.

The site map. Each site, the muscle that avulses it, and the movement that does it:

Why the injury is age-bound. The pelvic and hip apophyses generally appear in early-to-mid adolescence and fuse from the late teens into the early-to-mid twenties (17-25 years); the ischial tuberosity and iliac crest apophyses are typically among the last to fuse. That gives a window of vulnerability of roughly 11 to 25 years, peaking at 14 to 17, which is exactly the epidemiological peak.

Outside the window. Before the apophysis appears or after it fuses, the same explosive mechanism produces a muscle or musculotendinous strain, or a tendon avulsion such as the adult proximal hamstring tendon avulsion, rather than an apophyseal fracture. That is the single most useful framing for the age-versus-injury question.

The Risser sign. The iliac crest apophysis is the basis of the Risser sign of skeletal maturity (its ossification and excursion are covered in the Risser sign topic), which is useful for gauging remaining growth in the same patients.

Pelvic avulsions are described by site (the site map above) and by displacement, which is graded:

• Minimal (less than 1cm): conservative, with an excellent prognosis
• Moderate (1-2cm): conservative; there may be slight weakness
• Significant (greater than 2cm): consider ORIF, especially at the ischial tuberosity

Where the 2cm figure comes from, and why it is softer than it looks, is set out under Management.

History. An adolescent (13-17) feels a sudden pop and pain while sprinting, kicking or jumping.

Examination. There is point tenderness over the avulsion site: ASIS, AIIS, ischial tuberosity or iliac crest. Swelling and bruising are variable. The involved muscle is weak (hip flexion, or the hamstrings), stretching it is painful, and the gait is antalgic.

Radiographs. An AP pelvis is the first investigation. Apophyses look irregular normally, so compare with the contralateral side to avoid missing an avulsion, and look for a displaced fragment. Each fragment moves in a characteristic direction:

• ASIS: displaced inferolaterally
• AIIS: displaced inferiorly
• Ischial tuberosity: displaced inferiorly, by the hamstrings
• Iliac crest: multiple small fragments, the apophysis stripping off

CT is used if the radiograph is unclear, and to quantify displacement.

MRI is rarely needed. It is for soft tissue assessment and chronic cases, for the tumour mimic (see Differential Diagnosis), and for the occult avulsion.

The occult avulsion. An unfused apophysis is cartilaginous and radiolucent, so a small or purely chondral avulsion can be radiographically occult. In the very young, when the radiograph is normal but the clinical picture fits, use ultrasound or MRI.

Most heal without surgery. In the largest series 97% were managed non-operatively and only 3% had surgery. The ischial tuberosity is the site to recognise for surgical consideration: it carries the nonunion risk (4 of Schuett's 5 nonunions), while the AIIS carries the chronic-pain risk.

Conservative management.

1. Rest: crutches for comfort for 1-2 weeks
2. Ice: 20 minutes, 3-4 times daily
3. Analgesia: NSAIDs, paracetamol
4. Physiotherapy: gentle range of motion in weeks 1-2, progressive strengthening in weeks 2-6, sport-specific training in weeks 6-12
5. Return to sport: at 6-12 weeks, when pain-free and at full strength

The 2cm threshold. Displacement greater than 2cm at the ischial tuberosity is the classic teaching threshold, and it is a round number rather than a validated one. The sources carded below do not agree, and they are not measuring the same thing:

• Over 20mm is Schuett's nonunion-risk finding: displacement above it raised the odds of nonunion 26-fold. That is a statement about natural history, not an indication to operate: the same series managed 97% non-operatively.
• Over 15mm is Ghanem's surgical recommendation, argued on quicker return to sport.
• "Over 2cm" is the round teaching number that sits between them.

A prognostic threshold and an operative threshold have been conflated, and the operative one quoted in most textbooks is the larger of the two. Treat 2cm as the point at which the conversation about surgery becomes serious, not as an instruction, and weigh the athlete's demands, the site, and the family's tolerance of a possible delayed operation. Calderazzi's review is the honest summary: surgery buys a faster return to pre-injury sport and near-zero nonunion, at the cost of heterotopic ossification in about 9% against 1.8% managed conservatively.

Why fix the displaced ischial tuberosity. For the acute injury, surgery is reserved for displaced fragments (more than 2cm) in athletes who need an early return to sport, primarily at the ischial tuberosity. The case for it:

• Large displacement may lead to chronic hamstring weakness
• Painful nonunion or malunion
• Return to high-level sport

Other surgical indications (rare). A symptomatic nonunion at any site, and a large AIIS fragment causing impingement (see AIIS Subspine Impingement below).

Ischial tuberosity ORIF. A posterior approach, and the structure to protect is the sciatic nerve.

1. Positioning: prone or lateral
2. Incision: gluteal crease, which hides the scar
3. Dissection: identify and protect the sciatic nerve; identify the hamstring origin
4. Reduction: reduce the avulsed fragment to the ischial tuberosity
5. Fixation: 2 x 4.5mm cortical screws or cannulated screws; suture anchors if the fragment is small
6. Closure: layered

AIIS ORIF. Very rarely needed; the indication is a large fragment causing impingement.

1. Approach: anterior (Smith-Petersen or direct anterior)
2. Reduction: reduce the AIIS fragment
3. Fixation: small screws or suture anchors

After fixation. Crutches for 2-4 weeks, non-weight-bearing for the first 2 weeks after ischial tuberosity ORIF, with early gentle range of motion. Progressive strengthening physiotherapy begins at weeks 4-6, and return to sport comes at 12-16 weeks after surgery.

Results. More than 90% of conservatively treated patients return to full sport, and ORIF of the ischial tuberosity gives good outcomes. In the long term there are no significant issues if the injury is managed appropriately.

What happens. An AIIS (rectus femoris) avulsion can heal with exuberant callus or in a malunited, low-lying, prominent position, leaving a bony prominence at the AIIS/subspine region. In hip flexion this prominent AIIS can abut the femoral head-neck junction, producing extra-articular (AIIS/subspine) impingement, an entity distinct from the intra-articular cam and pincer femoroacetabular impingement covered in the dedicated FAI topics. It is the main reason a healed AIIS avulsion can still need surgery later.

Presentation. Months to years after the original injury, a young athlete reports anterior hip or groin pain and limited, painful hip flexion (a positive impingement provocation). Imaging (AP and false-profile radiographs, and CT/3D or MRI) shows the low or prominent AIIS abutting the femoral neck in flexion.

Management. Symptomatic cases respond to arthroscopic or open AIIS/subspine decompression, reshaping the prominent spine (Larson). This is why a large or markedly displaced AIIS fragment is one of the few situations in which to consider intervening on an otherwise benign injury.

Global epidemiology

• Predominantly adolescents aged 11-17 with open apophyses; male predominance (around 76% in the largest series).
• Sport profile is region-driven: soccer and gymnastics dominate in European data (Rossi), while sprinting/running and kicking dominate North American data (Schuett). Track and field, dance, martial arts and racquet sports also feature.
• Overall a benign, self-limiting injury — roughly 97% are managed non-operatively with excellent outcomes.

Guidance across societies (no dedicated guideline exists)

Registry note. Pelvic avulsion fractures are not tracked by arthroplasty/implant registries (NJR, AJRR, AOANJRR, SHAR) — these are non-implant, paediatric soft-tissue-bone injuries, so registry survivorship data do not apply.

High- vs limited-resource practice

• Well-resourced settings: ready radiographs, CT to quantify displacement, MRI for occult cartilaginous avulsion or tumour-mimic reassurance, and access to surgical fixation/arthroscopic subspine decompression when indicated.
• Limited-resource settings: diagnosis rests on a single AP pelvis radiograph and clinical examination; conservative management is the near-universal default and yields good results, with the main risk being missed diagnosis rather than under-operating.

Related pages: Proximal Hamstring Avulsion for the adult tendinous version of the ischial injury - the same muscle group failing at a different weak link once the apophysis has fused; Proximal Hamstring Injuries and Hamstring Injuries for the muscle strain this is misdiagnosed as, which is the commonest reason an ischial avulsion is missed; Femoroacetabular Impingement for the intra-articular impingement that subspine impingement must be distinguished from, and for the arthroscopic decompression Larson describes; Osgood-Schlatter Disease and Tibial Tubercle Fractures for the same apophyseal biology at the knee - traction apophysitis at one end of the spectrum and frank avulsion at the other; Physeal Injuries and the Salter-Harris Classification for why the growth plate is the weak link in the immature skeleton at all; Pelvic Ring Injuries for the high-energy pelvic trauma this benign injury must never be confused with; and Ewing Sarcoma and Osteosarcoma for the tumours that healing avulsion callus mimics on plain film - the reason an atypical or painful-at-rest lesion is imaged rather than biopsied blind.

There are no randomised trials and no formal society guideline for pelvic apophyseal avulsions; practice is built on retrospective series and expert consensus. Key unsettled questions:

• Operative displacement threshold. Commonly quoted cut-offs range from over 15mm (Ghanem) to over 20mm (Schuett nonunion data) to "over 2cm" as a teaching round number. The figure is a guide, not a rule — there is no level I/II evidence defining a single threshold.
• Which site predominates. Older radiographic series report ischial tuberosity as the most common site (Rossi), whereas the largest modern imaging-era series reports AIIS as most common (Schuett, 49%). Both are quotable; the discrepancy reflects era, referral pattern and imaging access.
• Surgery vs conservative for the displaced ischial tuberosity. Surgery offers higher return-to-pre-injury sport and lower nonunion but adds heterotopic ossification and operative risk (Calderazzi). The trade-off is individualised by athletic demand, displacement and fragment size.
• Acute fixation vs delayed/secondary surgery. Many displaced injuries can be treated conservatively first, reserving fixation/excision for symptomatic nonunion — but this risks a harder secondary operation through scar.
• Role of MRI/ultrasound. Useful for purely cartilaginous avulsions in the young (radiographically occult) and for tumour-mimic reassurance, but routine advanced imaging is not justified when the radiograph and history are clear.